This invention generally relates to an article transport system. More particularly, this invention relates to an article transport system in which the article is inspected for certain attributes as it is being transported. Specifically, this invention relates to a modified type of silent chain in which a center group of links may float relative to their assembly pins and wherein the links are held in a fixed location by a magnetic field in an inspection zone.
The basic purpose of the present invention is to provide a stable moving platform from which articles may be measured, while in motion on this platform, with accuracy and repeatability. The basic component of the present invention is a modified type of silent chain. The term "silent chain" is one which is well recognized in the mechanical arts, and describes a particular type of chain. An example of silent chain may be seen in Chapter 8 of The Standard Handbook For Mechanical Engineers by Bauneister and Marks. Basically, this type of chain is made up of a plurality of links, extending both longitudinally and transversely, which are held together with pins. The primary advantage of this particular type of chain is that chordal action is very much reduced. Chordal action is a variation in chain velocity which is due to a sprocket being a polygon as opposed to a cylinder of constant radius. Ordinary roller chain is subject to repeated variations in chain velocity as it is transported around sprockets. This variation in velocity makes it very difficult, if not impossible, to use conventional roller chain to provide a stable moving platform. As is known in the art, two pins normally hold a section of silent chain together and are allowed to move or roll relative to one another as the chain flexes to engage a sprocket. In so doing, the pins function as cams and cause a slight increase and a slight decrease in the pitch of each length as it engages the sprocket. This then allows compensation for the otherwise unavoidable velocity variations as the chain is moved around the sprocket. There are many variations in the types of pin used to hold the silent chain together. For example, both pins may be free to move relative to one another, or one of the pins may be fixed in position while the other pin rolls relative to that pin. Whatever the case, the basic principle involved is the same. Namely, there is relative motion of the pins which hold the links together in order to compensate for chordal action. Because of this ability of silent chain to compensate for chordal action, it is possible to have a very smooth running chain with minor variations in velocity. This would seem to allow use of such a chain as a stable moving platform from which measurements could be made. However, the conventional manufacturing tolerances are such that there are minor variations of a few thousandths of an inch from side to side and end to end in any section of silent chain. That is, the difference from side to side of a silent chain of six to eight inches in width may be several thousandths of an inch. Such a variation is of no concern in normal drive usage, but does create a problem when an article is to be measured to an accuracy of a few thousandths of an inch while it is moving on the chain. The present invention presents a solution to this situation and achieves a stable moving platform. This result is achieved by providing clearance between the assembly pins and the holes in a group of center links. These links are then pressed down to a fixed location and held there by a magnetic field as the chain transports articles through an inspection zone.